Patent GB-1261382-A (GB)
Title: Method and device for determining test symbols
IPC: G06F11/10
Filed: 1969-01-21
Abstract: 1,261,382. Calculating error detection symbols. PHILIPS ELECTRONIC & ASSOCIATED INDUSTRIES Ltd. 21 Jan., 1969 [24 Jan., 1968], No. 3339/69. Heading G4A. Symbols such as decimal digits or alphanumeric characters are provided with check digits which are combined in an error detecting process to give check characters. Each check digit is a number mod M. In the embodiment described a number M = 11 a system having p = 5 test symbols and m= 7 information symbols is used, and the test symbols are stored in five stores Sp 1 -SP 5 . In operation a number a 1  is added to store Sp 1 , the result is added to Sp 2 , the result added to Sp 3  etc., the stores originally containing zero, the final result being that all stores contain a 1 . In a second step a 2  is added to Sp 1 , the result added to Sp 2  etc. until the stores hold SP 1  a 1 +a 2  Sp 2  2a 1 +a 2  Sp 3  3a 1 +a 2  Sp 4  4a 1 +a 2  S P5  5a 1  + a 2  In a third step a 3  is added to Sp 1  and a similar procedure followed and in fourth, fifth, sixth and seventh steps a 4 , a 5 , a 6  and a 7  respectively are added to Sp 1  and a similar adding procedure followed until the stores contain Sp 1  a 1  + a 2  + a 3  + a 4  + a 5  + a 6  + a 7  SP 2  7a 1 +6a 2 +5a 3 +4a 4 +3a 5 +2a 6 +a 7  Sp 3  28a 1 +21a 2 +15a 3 +10a 4 +6a 5 +3a 6 +a 7  Sp 4  84a 1  + 56a 2  + 35a 3  + 20a 4  + 10a 5  + 4a 6  + a 7  Sp 5  210a 1  + 126a 2 + 70a 3  + 35a 4  + 15a 5 +5a 6 +a 7  Test symbols P 5 , P 4  are the contents of stores Sp 5 , Sp 4  respectively, the additions having been carried out modulo 11. Test symbol P 1  is formed in Sp 1  by adding the contents of Sp 4  to Sp 3  the result to Sp 2  and the result to Sp 1  giving Sp 1  120a 1 + 84a 2  +56a 3  + 35a 4 + + 20a 5  +10a 6 +a 7  Test symbol P 2  is formed in Sp 2  by then adding the contents of Sp 4  to Sp 3  and the result to Sp 2  and P3 is then the contents of Sp 3  after the contents of Sp 4  have been added thereto SP 2  315a 1 + 216a 2 + 140a 3 + 84a 4 + 45a 5 + 20a 6  +60 7  Sp 3  280a 4  +189a 2  +120a 3  +70a 4  +36a 5  +15a 6   +4a 7  The weights of the numbers a is given by the matrix when - 1!=1. The test symbols may be calculated by feeding a new digit to a store Sp 0  and arranging a switch S an adder Ad and stores Sp1-5 so that the switch couples two adjacent stores to the adder the sum being supplied to the second store. Alternatively the switch may have one arm and a register and a delay coupled to the adder may hold one of the quantities to be added (Fig. 1b, not shown). The mod M adder may comprise a series of mod M counters (Fig. 2) with pulses equal in number to the values aÁ plus an integral number of M pulses sufficient to transfer digits to counter Z5. When each counter receives M pulses it enables an input to the following counter. Numbers may be transferred from counter Z 4  to counters Z 3 -Z 1  by supplying pulses on the upper line. If only three test symbols are required the numbers may be supplied as above to provide test symbols in stores 2 and 3, these quantities may be read out and the contents of store 1 added to the contents of store 2 to produce the final test symbol.